Electroded cascade development system



Jan. 28, 1969 5 S E ET AL 7 3,424,131

ELECTRODED CASCADE DEVELOPMENT SYSTEM File d Sept. :50, 1964 Sheet of 5 FIG. 1

' INVENTORS GILBERT A. ASER BY I DAVID R. STOKES v "ZQZZaa-W Jan. 28, 1969 Filed Sept. 50, 1964 G. A. ASER ET AL ELECTRODED CASCADE DEVELOPMENT SYSTEM Sheet 2 Of 5 MOT-l0 INVENTORS GILBERT A. ASER DAVID R. STOKES A T TORNEYS Jan. 28, 1969 ASER ET AL 3,424,131

ELECTRODED QASCADE DEVELOPMENT SYSTEM I Sheet Filed. Sept. 30, 1964 //v VEN TORS GILBERT A.

Jan. 28, 1969 A, ASER ET AL 3,424,131

ELECTRODED CASCADE DEVELOPMENT SYSTEM Filed Sept. 30, 1964 Sheet 4 of 5 MOT-IO IN VEN TORS GILBERT A. ASER DAVID R. STOKES 8kg,

A TTORNEVS Jan. 28, 1969 G. A. ASER ET AL 3,424,131 ELECTRODED CASCADE DEVELOPMENT SYSTEM Filed Sept. 30, 1964 Sheet 5 of 5 20 SR-l r Ll t pl W T: ||5 v 60 CYCLE 7E'SR-2 3: REGULATED TR-l $2 T4 [L I T FIG. 6'

INVENTORS GILBERT A. ASER 19 DAVID R. STOKES A T TOR/V5 VS United States Patent 3,424,131 ELECTRODED CASCADE DEVELOPMENT SYSTEM Gilbert A. Aser, Rochester, and David R. Stokes, Fairport, N.Y., assignors to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Sept. 30, 1964, Ser. No. 400,489 U.S. Cl. 118-637 Int. Cl. B0510 5/02 3 Claims ABSTRACT OF THE DISCLOSURE This application is concerned with xerography and in particular the improvement of xerographic images by the elimination of toner particles from the image background.

In the xerographic process an electrostatic latent image is formed on a photoconductive insulating member and is developed or made visible by the attraction thereto of finely divided pigmented material. The most widely used automatic xerographic machines employ a photoconductive insulating member in the form of a cylinder with a horizontal axis and use the so-called cascade type of development in which a developer material is poured or cascaded over the surface of the zerographic cylinder. As is well known, cascade developer generally comprises a mixture of granular particles and much smaller pigmented resin powder particles, which electrostatically adhere to the larger particles. Since the smaller particles, called toner, are adherent upon the larger particles, called carrier, the developer mixture flows and otherwise behaves substantially as a granular rather than a powdered material. As the developer mixture is cascaded over the electrostatic latent image bearing xerographic drum, toner particles separate from the carrier and adhere to the drum in image configuration.

Presently available machines of this general character employ a xerographic drum having a diameter of about 15 inches and operate at a linear speed of about 20 feet per minute. Other machines having an 8-inch diameter drum are developed at surface speed of 10 feet per minute. It is obviously desired for some applications to increase the speed of operation of machines and to make greater speeds available with machines employing even smaller drums. As machine speed is increased with present developing apparatus, image density begins to fall oil? and other forms of image degradation appear which are associated with incomplete development. As the diameter of the xerographic cylinder is decreased, these effects set in at lower and lower speeds, since the time spent by any portion of the cylinder in contact with the flowing developer stream is progressively reduced.

Since the speed of the drum is increased, one solution to the problem is to enlarge the development area. Such a solution is set forth in U.S. Patent No. 3,105,770, issued Oct. 1, 1963, to Lehmann et al. Although this solution offers some advantages, it is unsatisfactory due to the long period of time in which the developer is in contact with the xerographic drum. It was found that the carrier, that is the beads, was depositing the toner on the xerographic drum and then subsequently picking the toner back up.

In drums which utilize high speed and a shorter development area, there is excessive background due to the fact that the carrier beads do not have the opportunity to pick up toner deposited in non-image areas, i.e., scavenge the background. The scavenging effect is presumably based on the theory that the heads will pick up loosely held toner, but will not pick up toner that is held by strong electrostatic charges.

The same problem exists in the electrostatic process that uses photoconductive paper, such as that having a coating of zinc oxide. In this process, the latent electrostatic image is formed directly on the photoconductive paper rather than on a xerographic plate. It is also desirable in this process to remove non-image material and to shorten the development area. Applicants have found that background toner, that is, toner particles being held on the xerographic plate by residual electrostatic charges, can be removed in a short development area by providing an electrical field above the xerographic plate of suflicient intensity to attract the toner from the plate surface. The toner which is held by the residual charge will when loosened by the movement or scrubbing effect of the carrier beads and will be attracted away from the xerographic plate by the field potential. The toner in the image areas is held by a much stronger electrostatic charge than the toner in the background areas and, if the field above the xerographic plate held within, sufficiently limits the electrostatic forces in the image areas will retain most of the toner in the image area.

It is therefore the primary object of this invention to eliminate toner material from the background of xerographic image bearing members.

It is also an object of the present invention to provide apparatus to eliminate background from an image bearing member by the removal of toner material from non-image areas.

Another object of the present invention is to improve xerographic apparatus to permit the use of a high speed xerographic drum that will produce a background-free image.

Another object of this invention is to provide a scavenging electrode which will eliminate background material from xerographic images.

Another object of this invention is to provide a scavenging electrode that will operate at all humidities.

These and other objects of the invention are attained by applying a suitable voltage to an electrode element positioned adjacent a developed electrostatic image bearing member.

For a better understanding of the invention as well as other objects and further features thereof, reference is bad to the following detailed description of the invention to be read in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic view of a xerographic machine of the type suitable for use with the present invention;

FIG. 2 is an enlarged left hand side view of the developing apparatus of the machine shown in FIG. 1;

FIG. 3 is an enlarged right hand side view of the developing apparatus of the machine shown in FIG. 1;

FIG. 4 is a front view of the developing apparatus shown in FIGS. 2 and 3 with parts broken away to show internal structure;

FIG. 5 is a sectional view taken along line 5-5 of FIG. 4; and

FIG. 6 is a schematic wiring diagram of a power supply and switch mechanism suitable for use with the present invention.

As shown schematcially in FIG. 1, the automatic xerographic reproducing apparatus comprises a xerographic plate including a photoconductive layer or light-re ceiving surface on a conductive backing and formed in the shape of a drum, which is mounted on a shaft journaled in a frame to rotate in the direction indicated by the arrow to cause the drum surface sequentially to pass a plurality of xerographic processing stations.

For the purpose of the present disclosure, the several xerographic processing stations in the path of movement of the dnum surface may be described functionally as fol lows:

A charging station, at which a uniform electrostatic charge is deposited on the photoconductive layer of the xerographic drum;

An exposure station, at which a light or radiation pattern of copy to be produced is projected onto the drum surface to dissipate the drum charge in the exposed areas thereof and thereby form a latent electrostatic image of the copy to be reproduced;

A developing station, at which a xerographic develop ing material including toner particles having an electrostatic charge opposite to that of the electrostatic latent image are cascaded over the drum surface, whereby the toner particles adhere to the electrostatic latent image to form a xerographic powdered image in the configuration H of the copy being reproduced;

A transfer station, at which the xerographic powder image is electrostatically transferred from the drum surface to a transfer material of a support surface; and,

A drum cleaning and discharge station, at which the drum surface is brushed to remove residual toner particles remaining therein after image transfer, and at which the drum surface is exposed to a relatively bright light source to effect substantially complete discharge of any residual electrostatic charge remaining thereon.

The charging station is preferably located, as indicated by reference character A. As shown, the charging arrangement includes a corona charging device 21 which includes a corona discharge array of one or more corona discharge electrodes that extend transversely across the drum surface and are energized from a high potential source and are substantially closed within a shielding imember.

Next subsequent thereto in the path of motion of the xerographic drum is an exposure station B. An optical scanning or projection system is provided to project a flowing image onto the surface of the photoconductive drum from a stationary original.

The optical scanning or projection assembly comprises a stationary copyboard which consists of a transparent curved platen member 22 such as, for example, a glass plate or the like, positioned on the exterior of the cabinet, which is adapted to support a document to be reproduced, the document being uniformly illuminated and arranged in light projecting relation to the moving lightreceiving surface of the xerographic drum. Uniform lighting is provided by banks of lamps LMPS arranged on opposite sides of the copyboard. Scanning of the document on the stationary copyboard is accomplished by means of a mirror assembly which is oscillated relative to the copyboard in timed relation to the movement of the xerographic drum.

The mirror assembly, which includes an object mirror 23 is mounted below the copyholder to reflect an image of the document through a lens 24 onto an image mirror 25, which in turn, reflects the image onto the xerographic drum through a slot in a fixed light shield 26 positioned adjacent to the xerographic drum surface.

Adjacent to the exposure station is a developing station C in which there is positioned a developer apparatus 30, constructed in accordance with this invention. A detailed description of the developer apparatus appears below.

Positioned next and adjacent to the developing station is the image transfer station D which includes a sheet feeding arrangement adapted to feed sheets of support material, such as paper or the like successively to the xerographic drum in coordination with the presentation of the developed image on the drum surface at the trans fer station.

The sheet feeding mechanism includes a sheet feed device adapted by means of vacuum feeders to feed the top sheet, of a stack of sheets on a tray 41, to rollers 42 cooperating with the belts of paper transport 44 for advancing the sheet sufficiently to be held by paper transport 44 which in turn conveys the sheet to a sheet registration device 45 positioned adjacent to the xerographic drum. The sheet registration device arrests and aligns each individual sheet of material and then in timed relation to the movement of the xerographic drum, advances the sheet material into contact with the xerographic drum in registration with a previously formed xerographic powder image on the drum.

The transfer of the xerographic powder image from the drum surface to the sheets of support material is effected by means of a corona transfer device 51 that is located at or immediately after the line of contact between the support material and the rotating drum. in operation, the electrostatic field created by the corona transfer device is effective to tack the support material electrostatically to the drum surface, whereby the support material moves synchronously with the drum. while in contact therewith. Simultaneously with the tacking action, the electrostatic field is effective to attract the toner particles comprising the xerographie powder image from the drum surface and cause them to adhere electrostatically to the surface of the support material.

Immediately subsequent to the image transfer station, there is positioned a stripping apparatus to paper pickotf mechanism 52 for removing the sheets of support material from the drum surface. This device, which is of the type disclosed in Rutkus et al. United States Patent 3,062,536, includes a plurality of small diameter orifices supplied with pressurized aeriform fluid by a suitable pulsator or other device. The pulsator is adapted to force jets of pressurized aeriform fluid through the outlet orifices into contact with the surface of the xerographic drum slightly in advance :of the sheet of support material to strip the leading edge of the sheet from the drum surface and to direct it onto an endless conveyor 55 whereby the sheet material is carried to a fixing device 60. At the fixing device, the transferred xerographic powder image on the sheet of support material is permanently fixed or fused thereto as by heat. After fusing, the reproduction is discharged from the apparatus at a suitable point for collection externally of the apparatus by means of the conveyor 65. In the embodiment shown, the reproductions are discharged from conveyor into a receiving tray 61.

The next and final station in the device is a drum cleaning station E, having positioned therein a corona preclean device 66, a drum cleaning device adapted to remove any powder remaining on the xerographic drum after transfer by means of a rotating brush 71, and a discharge lamp LMP-l adapted to flood the xerographic drum with light to cause dissipation of any residual electrical charge remaining on the xerographic drum.

To remove residual powder from the xerographic drum, there is disposed a cylindrical brush 71 rotatably mounted on an axle and driven by a motor, not shown. For collecting powder particles removed from the xerographic drum by the brush, there is provided a dust hood 73 that is formed to encompass approximately two-thirds of the brush area. To ensure thorough cleaning of the brush, a flicking bar 74 is secured to the interior of the dust hood adjacent the edge of the outlet duct 75 of the dust hood and in interfering relation with the ends of the brush bristles whereby dust particles may be dislodged therefrom.

For removing dust particles from the brush and dust hood, an exhaust duct 76 is arranged to cover the outlet of the dust hood, the exhaust duct being connected at its other end to the wall of a filter box 77 attached to the dust hood. A filter bag 78 is secured within the filter box, with the mouth of the filter bag in communication with the exhaust duct. A motor fan unit MOT-6 connected to the filter box, produces a flow of air through the filter box drawing air through the area surrounding the xerographic drum and the dust hood, the air entraining powder particles removed from the drum by the brush as the air flows through the dust hood. Powder particles are separated from the air as it flows through the filter bag so that only clean air reaches the motor unit.

Suitable drive means are provided to drive the drum, rotating mirror and sheet feed mechanism at predetermined speeds relative to each other, and to effect operation of the bucket-type conveyor and toner dispenser mechanism and the other operating mechanisms.

There is provided a frame for supporting the components of the apparatus formed by a base plate 10 supported on legs 9. Vertical outboard and inboard frame plates 11 and 12, respectively, are secured to base plate 10 in spaced relation to each other.

The xerographic drum 20 is mounted on a horizontal driven shaft and the drum is positioned between frames 11 and 12 with the major xerographic components of the machine mounted around the drum.

It is believed that the foregoing description is sufficient for the purposes of this application to show the general operation of a xerographic reproducing apparatus. For further details concerning the specific construction of the xerographic apparatus shown, reference is made to copending application, now US. Patent No. 3,301,126, filed concurrently herewith on Sept. 30, 1964, in the name of Osborne et al.

In order to effect development of the electrostatic latent image on the cylindrical xerographic plate, the developing system shown at station C includes a developer apparatus which coacts with the cylindrical xerographic plate or drum to form a development zone wherein the charged and exposed surface of the drum is developed to form a powder image of the original document.

Specifically, the developer assembly or apparatus 30 includes a box-like developer housing having side walls 201 and 202 and a sheet metal outer shell or cover 203 forming in the lower portion thereof a reservoir for developer material. As shown, the side walls 201 and 202 are formed with a concave edge portion in conformity with the shape of the xerographic drum to permit the developer housing to be positioned closely adjacent to the drum. Retaining plates 205, also formed to conform to the shape of the drum are secured to the side walls to compress seals 206 sandwiched therebetween into contact with the outer edge of the drum to form a substantially powder tight seal. An inclined bafile 208 is secured to the brackets 209 inside faces of the side walls and extends therebetween to prevent dust and air currents from circulating within the housing adjacent to the surface of the drum.

A suitable bucket-type conveyor is used to convey developer material from the reservoir portion of the developer housing to the upper portion of the developer housing from where it is cascaded over the xerographic drum. In the embodiment disclosed, the bucket-type conveyor consists of a series of parallel spaced buckets 212 secured as by rivets 213 to a pair of conveyor belts 214 wrapped around conveyor drive pulleys 215 and conveyor idler pulleys 216 secured on drive and idler shafts 217 and 218 respectively, to rotate therewith. Each of these pulleys has mounted therein pins 221 which engage the lugs on the timing belts 2.14.

Drive shaft 217 is journaled in the side walls and is adapted to be driven by a sprocket 226 secured to the outboard end thereof. Idler shaft is mounted in a similar manner. As the conveyor is operated by means of a sprocket 226, the buckets will pick up a charge of de veloper material previously placed on the bottom or sump portion of the casing and carry this material upward. As the buckets travel around the upper set of pulleys, the developer material will be discharged onto a guide plate designated generally as 230 which has a vertical surface 234. A slightly inclined baffie 235 adjustable both horizontally and vertically is attached to baffie 208, thereby forming with vertical surface 234 and inclined surface 233 a chute generally indicated as 236. The developer material material will be discharged onto the guide plate 230 wherein the developer will cascade down the chute 236. A lip 238 extends from the vertical surface 234 of baffle 230. This lip 238 deflects the developer contacted thereby. The distance between the surface 234 and 235 should be greater at the entrance or upper portion of the chute 236 than at the exit or lower portion. The purpose in this arrangement is to have a continuous flow of developer with no backup in the chute. The developer should cascade down the chute from the buckets 212 in a continuous path to the drum.

The entire developer assembly is positioned in a manner so that the bottom portion of the chute 236 is directed in a vertical position adjacent to the drum along a line substantially at approximately 60 off the horizontal in the direction of rotation of the drum.

With this arrangement of the guide plate 230 and the baffle 2 35, the developing material discharged onto the guide plate will fall by gravity along the surface 231, 232 and 233 of the guide plate picking up speed before it contacts the chute 236, at which time the developing material is directed in a vertical path toward the drum.

As the xerographic drum rotates, the developing material is discharged into contact with the drum at approximately 60 of the horizontal in the direction of rotation of the drum. The developing material will cascade over the drum and will eventually fall off or be thrown off the surface of the xerographic drum at a point which is in approximately the horizontal position. Most of the developing material will be thrown off the drum in a path tangential thereto.

An electrode 248, shown in the form of a bar, although it can be in the form of a wire or plate made of a suitable conductor material such as aluminum, is secured to insulating material 249 which is mounted on a bracket 250 secured to baffle 208. The contact 260 of the electrode is connected to a suitable power source to be described below. The developer material will therefore cascade down chute 236 over the xerographic drum past the electrode 248.

As shown in the drawings, particularly FIG. 5 and FIG. 4, the bar shaped electrode 248 is of a width to extend across the xerographic surface in the cascade development zone. It is of the length substantially shorter than the length of the cascade development zone and it is positioned in a central portion of the development zone to define an unelectroded cascade portion thereabove and an unelectroded cascade portion therebeneath. As can be seen in FIG. 5, the unelectroded cascade lengths above and below the electroded zone are of a length substantially longer than the electroded length defined by the bar-shaped development electrode. This permits unelectroded cascade development to occur above and below the electroded portion of the development zone.

Normally, the voltage of the exposed image on the xerographic drum will be in the range of 4501000 volts. For the most part, the voltage of the exposed image will be approximately 800 volts. The residual voltage in the non-image areas will be approximately 350 to 400 volts. It has been found that with these conditions a potential on the electrode 248 of approximately 2000 volts under conditions of low humidity, i.e., less than 50% relative humidity and 4000 volts under conditions of high humidity will produce optimum results with the electrode elements spaced A" away from the plate.

The range of potentials on the electrode 248 may be varied according to the characteristics of the final copy desired. That is, as the potential on the electrode is increased greater amounts of toner in the background areas will be removed or attracted to the electrode; however, greater amounts of toner in the image areas will also be attracted towards the electrode. As the potential on the electrode is decreased, less toner will be removed from the background area and also, less toners will be removed from the image areas. The field set up by the potential on the electrode 248 attracts toner to the electrode. The attraction to the electrode must be greater than the attraction of the 350-400 volt charge in the background areas of the plate. The potential necessary to produce a field that would overcome the electrostatic forces in the background areas is the lower practical limit of the potential On the electrode for effective results in scavenging the toner material from the background areas. The upper elfective limit to the potential on the electrode is that which will not create a field strong enough to attract large quantities of toner from image areas. However, the voltage on the electrode 248 must be kept below a level which would produce arcing between the electrode and various componentsof the machine or corona effect on the electrode.

It has been found that best results are obtained when the ratio between the voltage of the electrode and the background voltage is approximately to 1 under conditions of low humidity and to 1 under conditions of high humidity.

If the xerographic drum is positively charged the toner will be negatively charged and the charge on the electrode will be positive. It is possible, however to reverse all the charges, thereby using a negative voltage on the xerographic drum, a positive voltage on the toner and a negative charge on the electrode.

The spacing of the electrode 248 from the xerographic plate may be varied if the potential applied to the electrode is also varied to produce the desired results. The electrode shown herein is spaced A" from the xerographic plate and the electrostatic voltages are based on this A" spacing. However, as the electrode is moved further away from the plate the voltage may be increased to produce the same results, and conversely, as the electrode is spaced closer to the plate the voltage may be decreased. Actual physical limitations on the spacing between the electrode and the plate are determined by the size carrier beads used in the cascade process, the speed of the xerographic drum and the length of the development area. The electrode cannot be spaced too closely to the plate so that the carrier beads lodge between the electrode and the plate. Also, the electrode cannot be spaced so far from the plate that the voltage required to produce the desired results would produce arcing or corona effect. The A spacing has been found to be optimum for xerographic machines of the type described above.

Since the humidity to a certain extent appears to effect the development, a humidostat is provided to 'actuate a switch SW-l shown in FIG. 6 which is connected to a 115 volt 60 cycle regulated power supply whichvlsas dual terminals Tl and T2 connected to the primary of the transformer. The humidostat may be of the type made by Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., Part No. H66B-4219. The switch SW-l is in the position contacting T2 while the electrode normally operates at 2000 volts. The humidostat is set such that when the relative humidity is 50% or higher, it will actuate the switch SW-l which will contact terminal T1 and the electrode will then operate at 4000 volts. The terminals Tl and T2 are connected to a transformer TR-1.

At terminals T-3 and T-4 coming off the secondary of the transformer is a common type voltage doubler circuit. The circuit derives its name from the fact that its DC voltage output can be as high as twice the peak value of the AC input. The action of the voltage doubler can be described briefly as follows. On the positive half cycle of the AC input, i.e., when the upper side of the AC input line is positive with respect to the lower side, the upper rectifier SR-l passes current and feeds a positive charge into the upper capacitor 0-1. As positive charge accumulates on the upper plate of the capacitor, a positive voltage builds up across the capacitor. On the next half cycle of the AC input on the upper side of the line is negative with respect to the lower side. The lower rectifier SR2 passes current so that a negative voltage builds up across lower capacitor C2.

As long as no current is drawn at the output terminal from the capacitor, each capacitor can charge up to a voltage of magnitude E, the peak value of the AC input. It can be seen from the diagram that with a voltage of +13 on one capacitor and E on the other, the total voltage across the capacitors is 2E. Thus, the voltage doubler supplies a no-load DC output voltage twice as large as the AC input voltage. The resistance R1 serves as a bleeder for capacitor C1 and voltage dropping resistance and the resistance R2 serves as a bleeder for capacitor C2. The output is connected to the scavenging electrode 248, which is placed adjacent the drum 20 which is grounded.

To catch the developer material that falls from the xerographic drum so that it may be returned to the reservoir in the developer housing, an upper pickofl 241 is secured in a position to overlie a lower pickoff bar 242. Pickotf bar 241 is adjustably secured to bafllc 208 by a pair of brackets 244 while pickotf bar 240, is adjustably secured to the outer shell of the housing directly adjacent the drum. Any developing material not returned to the reservoir and not caught by either pickotf bar 241 or 2 52 is caught by a pan 243 held in place by a depending pan support 245 secured to the bottom portion of the outer shell of the develo er housing. As a supply of developing material accumulates in this pan, it must be manually removed by an operator and returned to the developer housing. This accumulation is very small.

The developer housing is mounted by means of a rod 251 that extends through the upper portion of the housing side walls 201 and 202. A mounting bracket 252 is attached to the outer shell or cover 203. The rod 251 rests on mounts 255 secured to the sidewalls of the machine housing. The rod 251 is held in place on mounts 255 by lever 256. The mounting brackets 252 rests on studs 253 which are secured to the machine housing. For removal, the housing is released by means of lever 256 and moved to the right by means of handles 254 which are secured to the top of the developer housing as shown in FIGURE 2. The housing rests on studs 253 and brackets 252. When the developer housing is to be removed, it is moved to the right. The developer housing may then be pivoted about the studs 253 for easy removal.

As the developing mixture is cascaded over the xerographic drum, toner particles are pulled away from the carrier and deposited on the drum to form powder images, while the partially denuded carrier particles pass off the drum into the reservoir. As toner powder images are formed, additional toner particles must be supplied to the developing mixture in proportion to the amount of toner deposited on the drum. To supply additional toner particles to the developing mixture, a toner dispenser 35 is used to accurately meter toner to the developer mixture.

The toner dispenser 35 is of the type that is old and well known in the art. Any one of a number of wellknown powder or granulated material dispensers may be used.

The toner dispenser 35 includes a hopper 263, the upper end of which is provided with flanges 264 by means of which the hop-per is supported on the outer shell of the housing. A hinged cover 265 is secured to the flange of the hopper to close the top of the hopper. A separate motor MOT-10, which is actuated by a timer, actuates the toner dispenser 35.

While the invention has been described with reference to the structure disclosed herein, it is not confined to the details set forth, and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

What is claimed is:

1. In xerographic development apparatus of the type having a cylindrical xerographic surface, a development electrode and means to cascade two-component developer including carrier granules and charged toner particles across the xerographic surface in a development zone, the xerographic surface having a latent electrostatic image and residual electrostatic background voltages in non-image areas, the improvement wherein the development electrode means comprises:

an elongated electrically conductive element of a width to extend across the xerographic surface in the cascade development zone,

said conductive element being positioned in a central portion of the length of the cascade development zone, said conductive element being of a length substantially shorter than the unelectroded length of the cascade development zone preceding it to permit unelectroded development to occur prior to movement of the developed portions of the photoconductive surface beneath the conductive element, said conductive element also being of a length substantially shorter than the unelectroded portion of the cascade development zone following it, and

means to apply to said conductive element a voltage of the same polarity as that on the surface to be developed at a potential several times larger than the residual background voltages of the latent electrostatic image to thereby remove toner particles previously deposited in the non-image areas of the xerographic surface.

2. The development apparatus as set forth in claim 1 wherein the Xerographic surface is drum shaped and further including means to continually move the xerographic surface through the development zone.

3. The development apparatus as set forth in claim 1 wherein said conductive element is biased to a potential between about 2 000 and about 4000 volts positive DC.

References Cited UNITED STATES PATENTS 2,573,881 9/1951 Walkup et al. 1l7l7.5 X 2,892,446 6/1959 Olden 1l717.5 X 2,952,241 9/1960 Clark et al. 11717.5 X 3,011,474 12/1961 Ulrich 11717.5 X 3,117,884 1/1964 Greig l1717.5 3,190,198 6/1965 Eicholn -1.7

WILLIAM D. MARTIN, Primary Examiner. EDWARD J. CABIC, Assistant Examiner.

US. Cl. X.R. 

